It is known for the single-lens reflex camera using interchangeable lens that, when photometric operation is done within the camera through an interchangeably mounted lens with the diaphragm of said lens being fully opened (so-called TTL photometry with fully opened diaphragm), using a resultant value by directly coupling it with an exposure meter or subjecting said value to an automatic exposure control operation would lead to a significant error between an exposure value to be set and said photometric value so that a photographing result with a proper exposure would not be obtained.
Such deviation or error is largely classified into the one due to optical characteristics of individual lenses interchangeably mounted on camera and the one due to specific position at which a light receiving element is located within the camera. The former is caused by a fact that the lens aperture stopped down by a diaphragm adapted to be preset by a diaphragm preset ring at a moment of photographing cannot produce an accurate multiple proportional variation of actual illumination intensity in the film plane corresponding to a multiple proportional variational variation of preset diaphragm value marked around the diaphragm preset ring. More specifically, individual interchangeable lenses are different in their lens materials, lens arrangements and assemblying modes, so that the illumination intensity in the film plane depends upon a light transmissivity characteristics, a vignetting effect peculiar to each lens, and particularly so-called aperture eclipse occurring with the fully opened diaphragm which substantially reduces the effective illumination intensity in the film plane when the lens aperture is held close to its fully opened diaphragm value. In consequence, the illumination intensity in the film plane is reduced in its effective value and varies in a mode far from said multiple proportional variation as the lens aperture approaches to the fully opened diaphragm value while the illumination intensity which is practically uniform and multiple proportionally varies substantially corresponding to the successive steps of preset values marked around the periphery of the diaphragm preset ring can be obtained within a range of relatively small lens aperture values, for example, less than a F-value of 5.6. Effective value of the illumination intensity in the film plane is substantially reduced at the fully opened diaphragm (the maximum lens aperture) and a resultant value of photometric operation carried out at this fully opened diaphragm cannot be utilized to obtain a proper exposure, since it would be impossible, from this resultant value, to produce a multiple proportional variation of exposure exactly or acceptably corresponding to the multiple proportional variation of successive preset values within the range of relatively small lens apertures and thereby to obtain an exposure time corresponding to a preset diaphragm value. Such error appearing in the film plane due to variation of illumination intensity depending upon individual interchangeable objectives is referred to herein as the illumination intensity error in the film plane.
The photometric element exposed to the light coming through the lens with fully opened diaphragm is preferably located just in the film plane so that said photometric element may be operatively coupled to an exposure meter or utilized for automatic exposure control to obtain a proper exposure. However, if photometric operation is actually performed just in the film plane, the photometric element would be a critical obstacle for film exposure in photographing and it would possibly lead to a solution that the photometric element is located not in the film plane itself but at a position which is equivalent to said film plane or causes little photometric deviation from the value which would be obtained from the photometric operation performed just in the film plane. Such photometric position equivalent to the film plane may be, for example, a position of a focussing plate on which the light reflected by a mirror is focussed and this position is, in fact, preferred in that this position is in a conjugate relationship with the film plane. However, this position would be an obstacle not only for the user's view through a viewfinder but also for the user's focus adjusting operation. Accordingly, the photometric element has usually been located, in most cases, on the light exit end surface of a pentagonal-dachkant-prism or similar optical system for reflection and inversion of light which is mounted on the focussing plate, particularly along the periphery of a viewfinder eyepiece so that a viewing therethrough should not be prevented. Even such position of the photometric element has been encountered by a problem that a photometric result is obtained as a value slightly lower or darker than the illumination intensity actually given in the film plane, since the position of the photometric element is deeper than the position of the focussing plate which is practically equivalent and conjugate to the film plane by a length of the optical reflection and inversion path defined by said prism and there occurs much or less light absorption by said prism. It will be obvious that such location of the photometric element has never achieved the intended photometric effect exactly equivalent to that obtained from the photometric operation performed just in the film plane and necessarily subjected to an error depending upon a focal distance of each interchangeably mounted lens. The error of this type is referred to herein as the illumination intensity error due to the position of the photometric element.
Thus, the respective errors as above mentioned inevitably appear as various quantities of deviation from the actual photometric values which should be obtained through the fully opened diaphragms no matter whether the objectives have the same value of their fully opened diaphragms or the same focal distance. (Details of these errors are illustrated and described in the specification of U.S. Pat. No. 3,486,434.)
With the photographic camera of so-called diaphragm regulation priority type, it is usual to transmit an information signal corresponding to a preset diaphragm value from the lens to a photometric instrument or its associated circuit or an exposure control circuit in the camera body by suitable mechanical means such as a cam (an example of such means is disclosed in Japanese Patent Publication No. 53(1978)-33064) or by suitable electrical compensator means such as a variable resistor control of special arrangement depending upon a diaphragm value actually preset so that the exposure control of high precision can be achieved with an effective compensation of the errors as mentioned above and thereby a proper exposure is given.
An object of the present invention is to provide a novel automatic diaphragm control system by which various error quantities of the previously mentioned two categories depending upon different objectives interchangeably mounted on cameras are effectively compensated and operation of stopping down is effected from the camera body to give a proper exposure.